This invention relates to optical indicators and more particularly to an optical indicator of the type wherein a decrease in the length of a strain member produces movement of an indicator element.
In many assembly operations where fastening devices are used it is desirable to inspect assembled fasteners to determine that they are under load and have not become loosened due, for example, to vibrations or other forces acting upon the fastener in use. The fastener may be, for example, a stud, a bolt, a rivet, or the like.
Several prior art techniques have been utilized for determining when a fastener has been properly taken up. The apparatus of this invention, however, is related to a structure for giving a visual indication of the fact that a fastener is not under load or has become loosened.
Fastener take up may be measured by determining either torque, stress or strain values in a fastener. Since torque, stress and strain are all interrelated, it is possible to determine the stress levels in a fastener knowing the strain of the fastener. Similarly, it is possible to determine torque values knowing the stress imparted to the fastener at any point. Thus, for purposes of illustration, the terms torque, stress and strain may be used interchangeably.
The device of this invention is termed a strain indicator because it is the decrease in length of the fastener that is being sensed in order to determine a decreased load on the fastener. The term strain indicator should not be considered as limiting since in its broadest sense the device of this invention may be considered a stress or torque indicator as well.